Optimization of a thermal-CVD system for carbon nanotube growth

We illustrate the optimization of the operation of a thermal chemical vapor deposition (CVD) system for the growth of carbon nanotubes (CNT). We have studied the deposition parameters using the Taguchi matrix robust design approach. The CVD system, which employs solid precursors (camphor and ferrocene) carried by nitrogen gas flow through a hot deposition zone, where the deposition of carbon nanostructures takes place, involves a large number of tunable parameters that have to be optimized.With the aim of getting the best configuration for the development of massive and well-oriented CNT carpets, the Taguchi method allowed us to improve our system leading to the growth of extremely long CNTs (few millimeters) at a high deposition rate (500 nm/s) and yield (30% in weight of the carbon precursors feedstock), which were characterized by electron microscopy.We found that the growth temperature had the most important influence on the CNT diameter, whereas the substrate tilt wit respect to gas flow did not influence their growth (i.e. CNTs grow on every side of the silicon wafer substrates, always normal to the substrate surface). The carrier gas flow and catalyst concentration both showed a secondary impact on CNT growth, though they showed a consistent correlation to the growth temperature.     

Purity and resistivity improvements for electron-beam-induced deposition of Pt

Electron-beam-induced deposition (EBID) of platinum is used by many researchers. Its main application is the formation of a protective layer and the “welding material” for making a TEM lamella with a focused ion beam thinning process. For this application, the actual composition of the deposition is less relevant, and in practice, both the mechanical strength and the conductivity are sufficient. Another important application is the creation of an electrical connection to nanoscale structures such as nano-wires and graphene. To serve as an electrical contact, the resistivity of the Pt deposited structure has to be sufficiently low. Using the commonly used precursor MeCpPtMe₃ for deposition, the resistivity as created by the basic process is 10⁺⁵–10⁺⁶ higher than the value for bulk Pt, which is 10.6 µΩ cm. The reason for this is the high abundance of carbon in the deposition. To improve the deposition process, much attention has been given by the research community to parameter optimization, to ex situ or in situ removal of carbon by anneal steps, to prevention of carbon deposition by use of a carbon-free precursor, to electron beam irradiation under a high flux of oxygen and to the combination with other techniques such as atomic layer deposition (ALD). In the latter technique, the EBID structures are used as a 1-nm-thick seed layer only, while the ALD is used to selectively add pure Pt. These techniques have resulted in a low resistivity, today approaching the 10–150 µΩ cm, while the size and shape of the structure are preserved. Therefore, now, the technique is ready for application in the field of contacting nano-wires.     

Focused helium-ion-beam-induced deposition

The recent introduction of the helium ion microscope (HIM) offers new possibilities for materials modification and fabrication with spatial resolution below 10 nm. In particular, the specific interaction of He⁺ ions in the tens of keV energy range with materials—i.e., minimal deflection and mainly energy loss via electronic excitations—renders the HIM a special tool for ion-beam-induced deposition. In this work, an overview is given of all studies of helium-ion-beam-induced deposition (He-IBID) that appeared in the literature before summer 2014. Continuum models that describe the deposition processes are presented in detail, with emphasis on precursor depletion and replenishment. In addition, a Monte Carlo model is discussed. Basic experimental He-IBID studies are critically examined. They show deposition rates of up to 0.1 nm³/ion. Analysis by means of a continuum model yields the precursor diffusion constant and the cross sections for beam-induced precursor decomposition and beam-induced desorption. Moreover, it is shown that deposition takes place only in a small zone around the beam impact point. Furthermore, the characterization of deposited materials is discussed in terms of microstructure and resistivity. It is shown that He-IBID material resembles more electron-beam-induced-deposition (EBID) material than Ga-ion-beam-induced-deposition (Ga-IBID) material. Nevertheless, the spatial resolution for He-IBID is in general better than for EBID and Ga-IBID; in particular, proximity effects are minimal.     

Present and future applications of magnetic nanostructures grown by FEBID

Currently, magnetic nanostructures are routinely grown by focused electron beam induced deposition (FEBID). In the present article, we review the milestones produced in the topic in the past as well as the future applications of this technology. Regarding past milestones, we highlight the achievement of high-purity cobalt and iron deposits, the high lateral resolution obtained, the growth of 3D magnetic deposits, the exploration of magnetic alloys and the application of magnetic deposits for Hall sensing and in domain-wall conduit and magnetologic devices. With respect to future perspectives of the topic, we emphasize the potential role of magnetic nanostructures grown by FEBID for applications related to highly integrated 2D arrays, 3D nanowires devices, fabrication of advanced scanning-probe systems, basic studies of magnetic structures and their dynamics, small sensors (including biosensors) and new applications brought by magnetic alloys and even exchange biased systems.     

Precise control over oxygen impurities in nano-crystalline silicon thin film processed with a low hydrogen dilution gas system at near room temperature

An atmosphere highly diluted with hydrogen is essential to increase the crystal fraction during formation of hydrogenated nano-crystalline (nc) or micro-crystalline (μc) silicon thin films via chemical vapor deposition (CVD). This hydrogen-rich process, however, hinders the ability for the material to find adequate use in micro-electronic devices due to contamination that results in oxygen-related problems such as donor-like doping, defect creation, or passivation. The use of neutral beam assisted chemical vapor deposition (NBaCVD), with a low hydrogen ratio (R = H₂/SiH₄) of 4, successfully deposits a highly-crystallized nc-silicon (HC nc-Si) thin film (TF) at near room temperature (<80 °C) and effectively reduces oxygen contamination by as much as 100 times when compared to conventional plasma enhanced CVD. During the formation of HC nc-Si TF via NBaCVD, energetic hydrogen atoms directly react with oxygen atoms near the surface of the nc-Si TF and remove the oxygen impurities. This is a completely different mechanism from the hydrogen-enhanced oxygen diffusion model. This technology meets the recent requirements of a high deposition rate and low temperature necessary for flexible electronics.     

Laser-induced damage of the optical films prepared by electron beam evaporation and ion beam sputtering in vacuum

The laser-induced damage characteristics and adsorption effects of organic contamination molecules of two high reflectors (HR) prepared by electron beam evaporation (EB) and ion beam sputtering (IBS) method at 1064 nm is investigated in vacuum. It is found that EB films show the performance degradation of laser induced damages in vacuum while for IBS film, seems to have no this effect, in comparison with air environment. In addition, EB coatings also have the strong affinity with organic contamination molecules, in contrast of IBS films. The results reveal that ion beam sputtering (IBS) method seem to be one of the favorite film deposition techniques of the optical films used in vacuum and space environments.     

苝四甲酸二酐有机单晶纳米结构的制备及形成机理的研究

在分子束外延(MBE)系统中, 利用物理气相沉积(PVD)的方法在阳极氧化铝(AAO)模板上制备了有机 染料分子苝四甲酸二酐(PTCDA)的不同纳米结构; 并使用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、 高分辨透射电子显微镜(HRTEM)以及选区电子衍射(SAED)技术进行了系统的研究. 结果发现, 当衬底温度(T_s)为330 ℃时得到的是纳米丝、针、带以及棒; T_s为280 ℃, 230 ℃, 180 ℃时得到的主要是纳米棒, 并且纳米棒的长度随T_s的降低而变短; T_s为50 ℃时只能得到连续的PTCDA薄膜. HRTEM以及SAED结果证实了纳米针与棒为单晶. 依据SEM结果, 提出纳米结构的生成主要受T_s以及衬底表面曲率的影响.     

Formation,Dynamics, and Characterization of Nanostructures by Ion Beam Irradiation

Ion beam irradiation is a potential tool for phase formation and material modification as a non-equilibrium technique. Localized rise in temperature and ultra fast (～10^?12 s) dissipations of impinging energy make it an attractive tool for metastable phase formation. As a matter of fact, a major component of materials science is dominated by ion beam methods, either for synthesis of materials or for its characterization. The synthesis of nanostructures, and their modification by ion beam technique will be discussed in this review article. Formation of nanostructures using ion beam technique will be discussed first. Depending on species (e.g., mass and charge state) and energy range, there are various modes for an energetic ion to dissipate its energy. The role of the electron will also be covered in this article as a basic principle of its interaction with matter, which is same as for an ion. By using a simple reactive ion beam or electron induced deposition, a secondary phase can be nucleated by ion beam mixing techniques, either by using inert gas irradiation or reactive gas implantation on any desired substrate. Nucleation of secondary phase can also be executed by electron irradiation and direct implantation of either negative or positive ions. Post implantation annealing processes are required for the complete growth of clusters formed in most of these ion irradiation techniques. Implantation processes being inherently a non-equilibrium technique, defects always have a role to play in phase formation, amorphization, and beyond (blister formation). When implanted with large energy, even electrons, one of the lightest charged particles, also manifest these properties. Electronic and nuclear energy losses of the impinging charged particle play a crucial role in material modification. Doping a nanocluster, however, is still a controversial topic. Some light will be shed on this topic with a discussion of focused ion beam.     

Influence of surface temperature and surface modifications on the initial layer growth of para-hexaphenyl on mica (0 0 1)

Ultra-thin films of para-hexaphenyl (6P) were prepared on muscovite mica (0 0 1) utilizing organic molecular beam deposition (OMBD) under well defined ultra high vacuum (UHV) conditions. The 6P growth characteristics were studied as a function of substrate temperature and substrate surface conditions. For the initial state of layer growth, thermal desorption spectroscopy (TDS) was used to verify the existence of a wetting layer. In this monomolecular continuous wetting layer, the molecules lie flat on the surface and are rather strongly bonded. For thicker films, in-situ X-ray photoelectron spectroscopy (XPS) in combination with (TDS) was applied to reveal the kinetics of the layer growth. Ex-situ atomic-force microscopy (AFM) was used to determine the film morphology. In particular, the influence of surface modifications (carbon contamination, sputtering) on 6P layer growth was investigated. XPS and low energy electron diffraction (LEED) were used to characterize the mica surface before the film deposition. TDS and AFM revealed a considerable change in film growth, from a needle-like island growth of flat laying molecules on top of the wetting layer (for the air cleaved mica) to terrace-like film growth of standing molecules, without a wetting layer (after surface modifications).     

Change in Graphene Electronic Properties in the Presence of Acetone Vapor

Changes in properties of graphene grown by chemical vapor deposition (CVD) with water adsorbate removal from the graphene–SiO₂/Si substrate interface using an organic material, i.e., acetone, are studied. It is found that acetone vapor suppresses grapheme structuring under low-intensity nanosecond laser radiation (wavelength λ = 532 nm). It is found that the electron work function in graphene decreases by ～0.2 eV, which is presumably due to a decrease in the water adsorbate layer thickness at the mentioned interface.     

Effect of accelerating voltage on crystallization of self-standing W-nanodendrites fabricated on SiO2 substrate with electron-beam-induced deposition

Self-standing W-nanodendrite structures were grown on SiO₂ substrate using an electron-beam-induced deposition (EBID) process with various accelerating voltages from 400 to 1000 kV. Effect of accelerating voltage on crystallization of the nanodendrites was investigated. The nanodendrites consisted of nano-sized grains and amorphous structures. The nano-sized grains were determined to be W crystal in BCC structure. The higher was electron beam accelerating voltage, the higher was crystallinity of the as-fabricated nanodendrites. It is suggested that high-energy electron irradiation enhances diffusion of W atoms in the nanodendrites, promotes crystallization of W grains.     

Synthesis of Different TiO_2 Nanostructures and Their Physical Properties

Titanium dioxide(TiO_2) nanosheet, nanorod and nanotubes are synthesized using chemical vapor deposition(CVD) and anodizing processes. TiO_2 nanosheets are grown on Ti foil which is coated with Au catalyst in CVD,TiO_2 nanorods are synthesized on treated Ti foil with HCI by CVD, and TiO_2 nanotubes are prepared by the three-step anodization method. Scanning electron microscopy shows the final TiO_2 structures prepared using three processes with three different morphologies of nanosheet, nanorod and nanotube. X-ray diffraction verifies the presence of TiO_2. TiO_2 sheets and rods are crystalized in rutile phase, and TiO_2 tubes after annealing turn into the anatase crystal phase. The optical investigations carried out by diffuse reflection spectroscopy reveal that the morphology of TiO_2 nanostructures influencing their optical response and band gap energy of TiO_2 is changed for different TiO_2 nanostructures.     

Biochemical and biomedical applications of multifunctional magnetic nanoparticles: a review

Nanotechnology offers tremendous potential for future medical diagnosis and therapy. Various types of nanoparticles have been extensively studied for numerous biochemical and biomedical applications. Magnetic nanoparticles are well-established nanomaterials that offer controlled size, ability to be manipulated by an external magnetic field, and enhancement of contrast in magnetic resonance imaging. As a result, these nanoparticles could have many applications including bacterial detection, protein purification, enzyme immobilization, contamination decorporation, drug delivery, hyperthermia, etc. All these biochemical and biomedical applications require that these nanoparticles should satisfy some prerequisites including high magnetization, good stability, biocompatibility, and biodegradability. Because of the potential benefits of multimodal functionality in biomedical applications, in this account highlights some general strategies to generate magnetic nanoparticle-based multifunctional nanostructures. After these magnetic nanoparticles are conjugated with proper ligands (e.g., nitrilotriacetate), polymers (e.g., polyacrylic acid, chitosan, temperature- and pH-sensitive polymers), antibodies, enzymes, and inorganic metals (e.g., gold), such biofunctional magnetic nanoparticles exhibit many advantages in biomedical applications. In addition, the multifunctional magnetic nanoparticles have been widely applied in biochemical fields including enzyme immobilization and protein purification.     

Synthesis of high-quality monolayer graphene by low-power plasma

The growth of high-quality graphene on copper substrates has been intensively investigated using chemical vapor deposition (CVD). It, however, has been considered that the growth mechanism is different when graphene is synthesized using a plasma CVD. In this study, we demonstrate a dual role of hydrogen for the graphene growth on copper using an inductively coupled plasma (ICP) CVD. Hydrogen activates surface-bound carbon for the growth of high-quality monolayer graphene. In contrast, the role of an etchant is to manipulate the distribution of the graphene grains, which significantly depends on the plasma power. Atomic-resolution transmission electron microscopy study enables the mapping of graphene grains, which uncovers the distribution of grains and the number of graphene layers depending on the plasma power. In addition, the variation of electronic properties of the synthesized graphene relies on the plasma power.     

Selective growth of ZnO nanowires on substrates patterned by photolithography and inkjet printing

Zinc oxide nanowires (ZnO NWs) were grown by a two-step growth method, involving the deposition of a patterned ZnO thin seeding layer and the chemical vapor deposition (CVD) of ZnO NWs. Two ways of patterning the seed layer were performed. The seeding solution containing ZnO precursors was deposited by sol–gel/spin-coating technique and patterned by photolithography. In the other case, the seeding solution was directly printed by inkjet printing only on selected portion of the substrate areas. In both cases, crystallization of the seed layer was achieved by thermal annealing in ambient air. Vertically aligned ZnO NWs were then grown by CVD on patterned, seeded substrates. The structure and morphology of ZnO NWs was analyzed by means of X-ray diffraction and field emission scanning electron microscopy measurements, respectively, while the vibrational properties were evaluated through Raman spectroscopy. Results showed that less-defective, vertically aligned, c-axis oriented ZnO NWs were grown on substrates patterned by photolithography while more defective nanostructures were grown on printed seed layer. A feature size of 30 µm was transferred into the patterned seed layer, and a good selectivity in growing ZnO NWs was obtained.     

Three-dimensional nanofabrication by electron-beam-induced deposition using 200-keV electrons in scanning transmission electron microscope

Attempts were made to fabricate three-dimensional nanostructures on and out of a substrate by electron-beam-induced deposition in a 200-kV scanning transmission electron microscope. Structures with parallel wires over the substrate surface were difficult to fabricate due to the direct deposition of wires on both top and bottom surfaces of the substrate. Within the penetration depth of the incident electron beam, nanotweezers were fabricated by moving the electron beam beyond different substrate layers. Combining the deposition of self-supporting wires and self-standing tips, complicated three-dimensional doll-like, flag-like, and gate-like nanostructures that extend out of the substrate were successfully fabricated with one-step or multi-step scans of the electron beam. Effects of coarsening, nucleation, and distortion during electron-beam-induced deposition are discussed. PACS 81.16.-c; 81.07.-b; 68.37.Lp; 81.15.Jj; 79.20.Fv     

Effect of precursors on the growth of carbon filaments onto clay surface

The successful growth of carbon filaments on two different precursors, i.e., the pristine sodium-montmorillonite (Na⁺MMT), which undergoes reflux at 100 °C (r-MMT), and the Na⁺MMT exchanged with Fe³⁺ ions (MMT(Fe)), was attained through chemical vapor deposition (CVD). The products obtained were characterized by X-ray diffraction, thermogravimetry, scanning electron microscopy, and transmission electron microscopy. Refluxing can make the Fe³⁺ ions in the octahedral layer of Na⁺MMT migrate to the interlayer and exchange with Na⁺ ions. Furthermore, through calcination at 500 °C, the Fe³⁺ ions migrate again to the surface of the clay layer and form iron oxides, which can serve as precursors for the deposition of carbon. Although r-MMT contained less iron than the MMT(Fe), the ultimate yield of carbon components grown was almost the same, indicating that the iron species in r-MMT possess higher catalytic activity. However, on the surface of r-MMT, CVD hardly generated carbon nanotubes with a clear hollow structure but that those with a carbon fiber structure instead.     

Electron beam induced formation of carbon nanorods

Electron beam induced formation of carbon nanorods was realized in situ under high resolution scanning electron microscopy (HRSEM). When a CVD deposited carbon nanotube sample was irradiated with an electron beam in an HRSEM, progressive etching of the sample, expanding of the nanotubes, and formation of additional nanorods have been observed. Transmission electron microscopy study revealed typical nanorods of 20 nm in diameter and with amorphous structure. The direct observation of the synthesis of nanorods under electron microscopy manifests the possibility of nano-machining of such nanomaterials using electron beams. This may lead to future integration and networking of nanostructures of different functionalities, which is crucial for nanotechnology.     

Electron beam induced surface activation: a method for the lithographic fabrication of nanostructures via catalytic processes

In focused electron beam induced processing (FEBIP), the very narrow electron beam of a scanning electron microscope or transmission electron microscope is used to locally modify matter on the nanometer scale. Recently, the family of FEBIP could be considerably expanded by the technique of focused electron beam induced surface activation (EBISA). In EBISA, the surface itself gets chemically activated by the impact of the electron beam without the presence of precursor molecules. In the second EBISA processing step, the surface is exposed to a precursor molecule which is then catalytically decomposed at the pre-irradiated/activated areas and eventually continues to grow autocatalytically upon prolonged precursor dosage. In this way, electron irradiation and precursor dosage are effectively separated. One of the advantages is that, due to the autocatalytic growth, the size of the corresponding nanostructures can be controlled by the precursor dosage and corresponding electron proximity effects can be omitted. Another advantage is the parallel processing of the pre-irradiated regions during precursor dosage. This bears the potential to significantly reduce the fabrication times for larger deposits compared to the classical electron beam induced deposition approach, in which precursor molecules are sequentially dissociated by the impact of the electron. The fundamentals and apparent further developments as well as the potential and challenges of the comparably new EBISA technique, and more general of catalytic effects in FEBIP are presented and discussed.     

Nanoscale electron beam-induced deposition and purification of ruthenium for extreme ultraviolet lithography mask repair

One critical area for the adoption of extreme ultraviolet (EUV) lithography is the development of appropriate mask repair strategies. To this end, we have explored focused electron beam-induced deposition of the ruthenium capping or protective layer. Electron beam-induced deposition (EBID) was used to deposit a ruthenium capping/protective film using the liquid bis(ethylcyclopentyldienyl)ruthenium(II) precursor. The carbon to ruthenium atomic ratio in the as-deposited material was estimated to be ~9/1. Subsequent to deposition, we demonstrate an electron stimulated purification process to remove carbon by-products from the deposit. Results indicate that high-fidelity nanoscale ruthenium repairs can be realized.